The invention relates to replacement metal gate transistors and methods for making the same.
Replacement metal gate transistors are made by removing a dummy gate of polysilicon from the transistor by etching to expose a gate trench. The gate trench is then filled with the metal that will constitute the gate in the finished device. Replacement metal gate (RMG) processes facilitate making the high-k gate dielectric/metal gate transistors needed for sub-50 nm MOSFET devices.
However, when a metal gate material is deposited into the gate trench from which the dummy poly-Si gate has been removed by etching, it can be difficult to fill the trench completely so that voids are not present. Additional process steps are conventionally used, such as “wetting” the gate trench with PVD titanium, followed by “seeding” the wetted trench with a thin CVD aluminum layer, and then at last filling the trench with aluminum by a reflow PVD technique. However these techniques can still result in insufficient gap-fill characteristics, as well to a resistivity increase due to alloying and a decrease in adhesion.
The conventional techniques for RMG formation therefore require additional process steps which are still not fully effective in eliminating voids between the replacement metal gate and the gate trench. Process time, device yield and device reliability are consequently adversely affected.